According to the present electrokinetic theory, all electrokinetic phenomenon (e.g., streaming potential, electrophoresis, sedimentation potentials, and electro-osmosis) are interrelated phenomenon and are based upon the fundamental electrochemcial concept of the "double layer." The double layer concept was first described by Helmholtz in the last half of the nineteenth century and has been modified by many others since. In essence, an electrical double layer surrounds any surface in contact with a liquid and consists generally of an immobile layer of ions next to the surface and a mobile layer of ions electrostatically in equilibrium with the ions in the immobile layer. The Encyclopedia of Electrochemistry edited by Hampell (Reinhold Publishing Corp. 1964) identified the double layer as follows.
"At any phase boundary there is always some redistribution of electrical charge because of the inhomogeneous field. This may be represented as two parallel sheets of charge of opposite sign known as a double layer. This name is retained even if the structure is more complex." PA1 E.sub.s is the streaming potential PA1 .zeta. is the zeta potential of the electrolyte/solid interface PA1 .mu. is the fluid viscosity PA1 k is the electrolyte conductivity PA1 L* is the distance between the pressure drop measuring points PA1 d is the internal diameter of the circular fluid conduit PA1 D is the dielectric constant
Charles Reilley in his article entitled, "Fundamentals of Electrode Processes", Treatise on Analytical Chemistry, edited by Kolthoff and Elving (John Wiley and Sons, Inc. 1963), Part I, Volume 4, Chapter 42, at page 2127-2129, discusses in greater detail the electrical double layer. In particular, the double layer is analogized to the assemblage of a plurality of charged layers at the liquid-solid interface.
Non-invasive measurement of internal body quantities usually involves detecting and measuring the effects the desired quantity has on the body environment around that quantity. For example, if the desired quantity to be measured is blood flow or blood velocity, it is possible to measure this quantity using the non-invasive techniques of: (1) ensonification; (2a) impedance plethysmograph, and the related approach of (2b) detecting the modulation of the amplitude of an electric signal; (3) and the generated electrical current resulting from blood (a conductive fluid) flowing through an impressed magnetic field. United States patents disclosing the foregoing techniques include the following patents arranged by the above grouping: (1) U.S. Pat. No. 3,830,223 to Beretsky et al; (2a) U.S. Pat. No. 3,835,839 to Brown and U.S. Pat. No. 3,835,840 to Mount; (2b) U.S. Pat. Nos. 3,823,706 and 3,689,393 to Davis and U.S. Pat. No. 3,131,689 to Rodler; (3) and U.S. Pat. Nos. 3,809,070, 3,759,247 and 3,659,591 to Doll et al. Each of these techniques utilize at least one pair of detecting electrodes placed on the body surface and a means for impressing an external field (e.g., (1) sound, (2) electrical or (3) magnetic) into the vicinity of the detecting electrodes or with the detecting electrodes. Other techniques use the relatively expensive and potentially hazardous conventional x-ray and related electromagnetic radiation imaging methods.
The foregoing techniques are but a few of many different approaches. Each technique and approach has its advantages and disadvantages. With respect to a method of evaluating strictures in blood vessels, such as atherosclerotic lesions, none of the aforesaid patents disclose satisfactory methods or apparatuses. Usually the electrodes are not mobile enough and are not sensitive enough so that the small change in blood flow characteristics around such a stricture can be detected. Thus there is a need for a device and a method for the detection characterization and quantification of strictures in conduits containing a flowing conductive fluid such as atherosclerotic lesions in blood vessels.